The present invention relates generally to an accessory drive assembly and, more specifically, to a torque-limiting accessory drive assembly for the accessory drive system of an engine of a motor vehicle.
In automotive applications, engine torque and speed are translated from a prime mover, such as an internal combustion engine, to one or more wheels through the transmission in accordance with the tractive power demand of the vehicle. In addition to supplying power to move the automotive vehicle, the engine drives a variety of accessories, such as; the power steering pump; air conditioning compressor; cooling fan; water pump; alternator; and any other engine driven accessory. These various accessories are usually operatively belt driven via one or more pulley systems deriving their power source from an engine power take-off, typically a crankshaft pulley. In conventional accessory drive systems engine power is constantly supplied by the crankshaft pulley to the belts. Some of the driven accessories are fitted with clutch assemblies to lessen the mechanical load and drain on engine power when they are not needed (e.g., air conditioning compressor and cooling fan). However, clutches have yet to be employed at the power take-off from the engine that drives such accessories.
As certain drivetrain technologies have developed, there has become a need in the art to selectively interrupt power to the driven accessories as well as selectively initiate or return power to the driven accessories. One example of such a situation involves the use of automated manual transmissions in automotive applications. These types of transmissions automate the moving of shift forks and clutches so that the operator is not required to manually disengage the engine from the transmission by depressing a clutch pedal nor by moving a shift lever to change gears. As in more conventional transmissions, during a transmission upshift, engine speed is governed by throttle position while transmission speed tends to remain essentially constant. This is due to the fact that the transmission remains engaged to the driving members and wheels, and these parts will initially sustain transmission speed then, being subject to frictional and drag forces, will slowly decrease transmission speed accordingly, the longer the transmission is decoupled from the engine. Once the next gear is selected, the transmission and engine speeds must be synchronized. The gear change upward will cause the transmission to have a lower input speed requirement. As the clutch mechanism is re-engaged the engine is commanded to a lower throttle setting to match the required input speed to the transmission. This approach relies on the slow or unpredictable loss of engine speed by either mechanical factors or a driver input to the throttle setting and while this is generally accepted in the related art, it is inefficient. When the engine and transmission speeds are relatively the same, the clutch is engaged, recoupling the engine to the transmission and delivering torque to the drivetrain. The process works but has room for improvement.
Additionally, automated manual transmissions suffer from the problem of noticeable torque interrupt during shifting, unlike that which occurs in a fully automated transmission or which can be compensated for in a fully manual transmission. This phenomenon adversely affects the xe2x80x9cfeelxe2x80x9d of the vehicle during shifting and is undesirable from an operator""s viewpoint. Beyond the feel of a transmission shift, other adverse effects of a delayed and hard torque transfer can damage or reduce the efficiency of the vehicle""s accessory components. More specifically, when conventional accessory drive systems are employed excessive mechanical forces and stresses are often transferred to the accessories through the belt drive. This is due to the fact that, in conventional systems, the accessories are constantly connected and constantly driven. In such systems commonly employed in the related art, when the engine speed increases or decreases sharply, a quick change in torque is applied to the accessory components. This spikes, or sharply peaks, the load on the various accessories. These rapid load shifts can cause excessive or rapid wear in accessory bearings and the belt materials as well. Therefore, there is a need in the art for a device that can limit the torque transfer from the engine to the accessories.
Alternatively, it is often desirable to maximize power at vehicle launch. The engine driven accessories noted above have the undesirable effect of draining power that could otherwise be delivered to the drivetrain in this operating mode. Thus, there remains a need in the art for a device that selectively interrupts power to the driven accessories as well as selectively initiates or returns power to the driven accessories in an automotive application.
The present invention overcomes the disadvantages and drawbacks in the related art as a torque-limiting accessory drive assembly including a pulley adapted to transfer torque from the power take-off of an internal combustion engine to an accessory drive system, and a torque limiting device operatively connected to the pulley. The torque limiting device is adapted to limit the maximum amount of torque transferred from the power take-off of an internal combustion engine to the pulley. The assembly also includes a one-way clutch assembly operatively interposed between the engine power take-off and the torque limiting device. The one-way clutch assembly acts to selectively transfer torque from the power take-off to the torque limiting device thereby providing motive force to the pulley and vehicle accessory drive, and acts to selectively interrupt torque transfer at predetermined times.
The torque-limiting accessory drive assembly of the present invention therefore overcomes the disadvantages of conventional designs in two ways. First, the torquelimiting accessory drive assembly of the present invention is selectable in its engagement and disengagement, thereby allowing controlled interruption of the engine torque supplied to the accessories. Second, the multi-plate friction clutch provides for selective adjustment of the maximum torque transfer capacity. Furthermore, it does this without the need for supplying hydraulic power, which is the typical mechanism used to apply multi-disc friction clutches known in the art. The wrapped spring type, one-way clutch of the present invention can operate at high speeds over a wide range of temperatures. In addition, by using the one-way, spring clutch technology, the torque-limiting accessory drive assembly of the present invention is small and lightweight, having long life with an efficiency that is virtually unchanging.